The present invention relates to a spindle assembly for such use as an electrolytic grinder or others, in which a rotatable spindle transmits sufficient electric current to the electrode on the spindle for electrolytic working the current, being supplied by a stationary brush member.
The electrolytic grinder has a metal-bonded grinding wheel as the negative pole and a work as the positive pole close to the grinding wheel, between which electrolyte flows through, conducting electric current from the work to the grinding wheel, so that the surface of the work is gradually dissolved to be removed by electrolysis reaction. It is usual for supplying the desired electric power to the rotating grinding spindle which has the metal-bonded grinding wheel, to use a slip-ring or electric current collector on the spindle and brushes in contact with the slip-ring or electric current collector.
A device having an electric power collector fixedly mounted on the rear end of the spindle and a static plug brush, an end surface of which is in contact with the collector end surface, has been in use. The device of this kind is superior to that of a slip-ring and a pair of brushes radially in contact with the slip-ring periphery, in that it possesses larger contact area of the brush with larger current capacity and in compactness.
But, in this kind of device there tends to occur dynamic pressure in the gap between flat friction surfaces of the electric collector and the brush, decreasing the contact pressure and bringing poor electric conductivity to the device. Furthermore, in case of supplying a cooling oil-mist to the brush device, it penetrates into the gap between the friction surfaces, making an oil film of non-conductivity therein.
For avoiding dynamic pressure and oil film in the gap of the friction surfaces, conventional brushes are provided with radial grooves on their friction surfaces. But, the grooves become soon filled with wear grain material of the brush and the material combined with oil will be formed into electric non-conductive film between the friction surfaces after a short-hour operation, by reason of so high temperature caused by electric current heating and friction heating accompanying poor air circulation and wear material rejection.
In order to prevent current leak and to ensure operators safety, it is necessary to insulate the spindle from the other members of the assembly. In the conventional apparatus, synthetic resin sheets have been inserted between the bearing holders and the spindle housing, fastening the sheets with them, or synthetic resin coated on the bearing holders which are fixed on the spindle housing.
In these prior art techniques, it is difficult to completely eliminate gap or space between the bearing holders and the spindle housing. In addition, it is necessary for safety to make the insulating sheet or coated film rather thick. Poor rigidity of the spindle results from these gap and thick sheet. It is another defect of the prior art that dielectric breakdown is apt to occur at the gap because of electrolyte penetration. Moreover, precise position assembling of the spindle is very difficult.
Ceramic insulators are also known, but are brittle and of bad machinability, resulting in lowering of the spindle accuracy.